Thermoplastic and thermosetting polymers and resins are widely used in a variety of applications, and can be particularly useful as a component of a reinforcing composite structure. For example, phenolic/paper, epoxy/paper, epoxy/cotton, epoxy/glass fabric and several other types of reinforced laminates are presently used by the electrical industry as wire and cable insulation and as components in various electronics applications, particularly as the "board" component in conventional printed circuit boards. Each of these materials varies in usefulness depending upon cost, dielectric properties, and operating temperatures.
Reinforced composites are also used as structural components for various moldable structures, including sheet molding compositions and the like as are used to fabricate automotive body parts, sports equipment, furniture, medical products, agricultural products, industrial products, toys, containers, appliances and the like.
Thermoplastic polyolefins, because of their moldability and other properties, are widely used in molding applications, such as in the formation of sheet molding compositions. In addition, by virtue of their hydrocarbon structure, thermoplastic olefin polymers would be particularly useful for electronic applications requiring low dielectric loss properties and good electrical insulation were it not for their low melting points. Thermoplastic polymeric materials typically melt below temperatures required for soldering and other manufacturing steps in printed wiring board manufacturing and applications. Polyethylene would be particularly suitable for applications requiring minimal electrical loss in terms of dissipation factor and loss tangent due to radio-frequency energy interference. In such highly demanding electronic environments, the epoxies and like thermoset polymers as are conventionally used in electronic applications provide increasingly unsatisfactory to inadequate protection against electrical loss.
Polyolefins have been combined with a suitable reinforcing substrate, such as a glass fabric, to produce laminates having some of the thermosetting properties of materials like the epoxy laminates. The use of such a reinforcing substrate can improve the physical properties of the resin and produce a reinforced polyolefin possessing good dimensional, tensile, flexural, bursting and tear properties. Nevertheless, in many applications, reinforcing composite structures need to be flame-retardant, drip resistant, and have low thermal expansion properties, particularly in the Z-axis direction. Because polyolefins have a relatively low softening point, poor heat resistance and high Z-axis thermal expansion, the polyolefins are unsuitable for many applications requiring the use of heat, for example, in the fabrication of circuit boards, where the addition of metallic foils followed by some form of soldering is required. As a result, although polyolefin laminates have the desired electrical or other characteristics and adequate strength, exposure of the laminates to elevated temperatures can generally result in thermal distortion and delamination of the laminate. Further, because of their thermoplastic properties, polyolefins can drip when heated; and due to their hydrocarbon-based chemical structure, they are not flame retardant.
Polyolefins can be crosslinked and then combined with a reinforcing substrate such as a glass fabric to address issues of structural stability under high temperature conditions. Crosslinking can be achieved by irradiating sheets of crosslinkable polyolefin with high energy electrons after extruding the sheets and prior to forming a laminate. Reinforced polyolefin laminates of this type have been fabricated by combining a thermoplastic olefin polymer such as polyethylene crosslinked by irradiation and a reinforcing substrate such as a woven glass fabric by applying the polymer to the reinforcing substrate and then heating the laminate to fuse the resin to the substrate. Irradiation of the polymer sheet material and the subsequent assembly of the sheet material into a laminate by combining the sheet material with a reinforcing substrate and heating the combination under pressure to achieve consolidation is, however, expensive and difficult to maintain a consistent treatment in continuous production.
Crosslinked polyolefin laminates can also be chemically achieved by physically blending a free radical initiator with polyethylene pellets. The formation of a laminate structure using such polyethylene pellet/initiator blends under appropriate high temperature conditions initiates the crosslinking reaction. Such crosslinking has, however, produced a non-homogenous variable dielectric polyolefin material. This can be particularly disadvantageous in microelectronic applications, where polymer consistency is critical to achieving a structure, such as a circuit board substrate, having highly uniform dielectric properties.
U.S. Pat. Nos. 4,395,459 and 4,292,106, issued Jul. 26, 1983, and Sep. 29, 1981, respectively, to Herschdorfer and Vaughan, are directed to a process for producing reinforced laminates from crosslinkable thermoplastic olefin polymer materials and to the resultant reinforced laminate products. A mixture of a crosslinkable thermoplastic olefinic polymer containing a free radical initiator which can be subsequently activated to crosslink the polymer is extruded to form a continuous non-reinforced film. The film can thereafter be combined with a reinforcing substrate to form a coated fabric or prepreg, and in turn, the prepreg can be subsequently heated under pressure to a temperature above the free radical initiator activation temperature to initiate reaction with the polymer and to effect substantially complete crosslinking of the polymer. This converts the thermoformable prepreg into a permanently shaped, i.e., non-thermoformable structure. The resultant reinforced laminates are stated to exhibit excellent physical properties, particularly flexibility, high strength and resistance to heat, as well as excellent electrical properties, good chemical and solder resistance and none of the drip characteristics normally associated with thermoplastics.
Despite the highly desirable properties reported in the '459 and the '106 patents, the laminates disclosed therein have not been commercially manufactured because of difficulties associated with the processing techniques disclosed therein, and the resulting shortcomings of the products. In particular, due primarily to the heat activatable crosslinking properties of the polymer melt, processing parameters of heat and shear must be rigidly controlled during the film formation step to avoid premature crosslinking of the polymer and the accompanying permanent hardening of the polymer within the interior of the extrusion apparatus and dies. The lack of substantial flexibility to vary polymer temperature, in turn, greatly limits the availability of traditional processing controls in which different temperature conditions are employed to control the rheological properties of the melt during film formation.
For example, when molten polymer is fed into the central portion of a film forming die, the polymer extruded from the edge portions of the die must travel along a longer path prior to reaching the exit point of the die as compared to polymer extruded from the middle of the die. If the polymer having a longer residence time in the die is cooled to a greater extent than polymer experiencing a shorter residence in the die, the viscosity of the different polymer portions will be different with the result that film properties, particularly thickness, will be variable. Although die feed and cavity structures can be designed to minimize any polymer residence differences, it is difficult to completely eliminate dead spots within the die and/or ensure that all polymer traveling through the die will have the same heat history.
In conventional extrusion processes, i.e., melt extruding polyolefin polymers without a crosslinking agent, these difficulties can be addressed by increasing polymer temperature, thus lowering viscosity, and/or by increasing polymer throughput. However, the limited temperature flexibility available in the extrusion of crosslinkable polyolefin polymer/free radical initiator mixtures precludes substantial reliance on these traditional remedies. For example, when a heat activatable crosslinking agent is present in the resin, the freedom to operate the extrusion process within a wide range of temperatures (i.e., at higher temperatures) is not available. This can be significant in that even small temperature changes can greatly impact polymer viscosity, and thus greatly impact the ability to produce uniform polymer films. Even with improved temperature controls, it can be nearly impossible to operate a melt extrusion process so that the polymer has the same heat history and behaves the same across a film width.
As film thickness increases, the differentials in polymer heat history, and resulting differences in film thickness, are less problematic. However, for many applications, such as printed circuit boards, it is advantageous to significantly decrease film thickness to provide desired polymer-to-substrate ratios. However, as film thickness decreases, particularly as film width increases, the possibilities for differentials in polymer temperature history is significantly affected. Further, as both film thickness decreases and film width increases, any such differences in the heat history of the polymer are significantly magnified. As a result, not only is non-uniform film thickness a problem, but voids in the film can also occur.
As a result, those skilled in the art have not produced, commercially or experimentally, a relatively low thickness, large width composite sheet products, i.e., sheets having a thickness of about 10 mils (0.010 in) or less and a width above about one foot, and higher, in accordance with the Herschdorfer and Vaughan '459 and '106 patents. Similarly, no films or other sheets having an average thickness substantially less than about 5 mils (0.005 in) have been produced experimentally or commercially that avoid substantial variations in film thickness of greater than about plus or minus 20% based on average film thickness across the width of the sheet for films of a width exceeding about one foot, and higher.
From a commercial standpoint, the width of the composite sheet can be important for numerous reasons. These reasons include the ability to use currently available converting equipment to form articles or components from the composite, and economic penalties associated with small scale manufacturing operations which necessarily result from commercial production of end products or components from low width composite sheets.
Further, in high value-added end use applications, particularly electronic and other technically demanding applications, the uniformity of polymer distribution in the final laminate and/or the ratio of resin to reinforcing substrate can be important factors in determining the properties, and thus the usefulness, of the end product.
If too much or too little polyolefin is present in the structure relative to reinforcing materials such as glass or other fibers, or if the distribution of the polyolefin throughout the composite cannot be precisely controlled, important product properties can be sacrificed. Thus the strength of the composite can suffer, or in the case of multi-layered laminates, the laminate stability and the layer-to-layer bonding strength can be unsatisfactory. Moreover, these same shortcomings can prevent products from meeting dielectric and insulation/conductivity property specifications in the case of electronic components.